When you use the button by entering a keyword phrase and hitting the “I’m Feeling Lucky” button, Google sends you straight to the very first webpage in their results that matches that phrase.

Spammers have apparently found that they can embed links like this one for Natural Search Blog to the Google I’m Feeling Lucky functionality, and their emails will bypass filters that would automatically catch lots of blacklisted and spammy-looking URLs. Email filters allow links to search engine results through because many people may genuinely send such links to one another.

So, how will this get fixed? I’d expect that Google may have to lock down their “I’m feeling lucky” functionality so that it only works for users referred directly from the Google homepage, and from the Google Toolbar (if the user has enabled the I’m Feeling Lucky button on the toolbar).